In recent years, there has been used a small piezoelectric/electrostrictive element for a power source of a HD (hard disk) head, a motor of a cell phone or a digital camera, a power source for ejecting ink of an ink-jet printer, or the like.
In a film type piezoelectric/electrostrictive actuator, it is effective to make the thickness of the film thin in order to obtain a large displacement in the case of using the actuator in the Same driving electric field, and the thickness can be reduced to about 10 μm or less. A piezoelectric/electrostrictive ceramic composition used for such a film type piezoelectric/electrostrictive actuator is required to have a characteristic that the increase rate of the displacement is not decreased even when the electric field becomes higher.
In contrast, there is a laminate type piezoelectric/electrostrictive actuator having a thickness of about 100 μm, and the driving electric field is lower than that of a film type piezoelectric/electrostrictive actuator. Such a laminate type piezoelectric/electrostrictive actuator is required to have a characteristic of having a large displacement upon applying a low electric field while it is not requested to have the characteristic of having a large displacement upon applying a high electric field.
As a piezoelectric/electrostrictive ceramic compositions used for such a laminate type piezoelectric/electrostrictive element, there have conventionally been known compositions having composition formulae of Pb(Mg1/3Nb2/3)O3—PbTiO3—PbZrO3 (sometimes referred to as PMN-PZT type), Pb(Ni1/3Nb2/3)O3—PbTiO3—PbZrO3 (sometimes referred to as PNN-PZT type), and Pb(Zn1/3Nb2/3)O3—PbTiO3—PbZrO3 (sometimes referred to as PZN-PZT type) (see, e.g., JP-A-2001-302349 and JP-A-2004-115346).
A piezoelectric/electrostrictive actuator using a conventional piezoelectric/electrostrictive ceramic composition satisfies a required displacement and shows high insulation resistance value at an early stage of manufacture. However, in the case of using the piezoelectric/electrostrictive actuator repetitively, the insulation resistance value of the piezoelectric/electrostrictive actuator may decline over time. In particular, in the case of using the piezoelectric/electrostrictive actuator under a high-humidity environment, the insulation resistance value may decline to a great extent. Such a decline in the insulation resistance value under a high-humidity environment is a problem to be solved from the viewpoint of high reliability in recent years.